Due to enhanced worldwide concern about ecologically harmful substances, anticorrosive pigments need be free from a harmful heavy metals such as Pb and Cr. Zinc phosphate and other zinc compound-based pigments have been one of such ecologically friendly anticorrosive pigments. However, it has been reported that zinc may retard the growth of aquatic organisms and, therefore, is becoming a target of ecological concern. Zinc phosphate and zinc oxide are designated as a ecologically harmful substance in some European countries for this reason. Aluminum dihydrogen tripolyphosphate is known as a base component of anticorrosive pigment containing neither heavy metals nor zinc. Since this compound is a solid acid, the formulation of this pigment adversely affects the storage stability of the paint containing the same, particularly when formulating into a water-based paint which is becoming popular today for the purpose of reducing the emission of VOC to the atmosphere.
Japanese Patent No. 3,186.908, assigned to Tayca Corporation, discloses an anticorrosive pigment intended to be formulated into a water-based anticorrosive paint. The pigment comprises aluminum dihydrogen tripolyphosphate which has been treated to replace two protons with ammonia, combined with zinc dioxide and/or oxide or hydroxide of an alkaline earth metal. Replacement of protons of the starting aluminum dihydrogen tripolyphosphate causes to substantially diminish its acidic nature and is effective for the prevention of the adverse effect on the storage stability when incorporated into a water-based anticorrosive paint. The treatment with ammonia requires, however, additional steps and thus additional costs.
Japanese Patent No. 3,440,325, also assigned to Tayca Corporation, discloses another anticorrosive pigment to be incorporated into a water-based anticorrosive paint, which comprises aluminum dihydrogen tripolyphosphate and magnesium hexasilicate. This pigment finds use in a water-based anticorrosive paint comprising vinyl chloride-vinylidene chloride copolymer emulsions.
A need exists, therefore, for an anticorrosive pigment composition free from harmful heavy metals and zinc, which does not deteriorate, when incorporated into a water-based paint formulation, the storage stability of the paint while exhibiting a sufficient anticorrosive performance at least as high as the known zinc phosphate type anticorrosive pigment.